Patent GB-1072363-A (GB)
Title: Light measuring system
IPC: G01J1/16
Filed: 1964-06-03
Abstract: 1,072,363. Photo-electric colour measurement. G. BERGSON. June 3, 1964 [June 24, 1963], No. 23054/64. Heading G1A. The invention is concerned with apparatus for determining the colour of a test sample either by transmitting light through it or reflecting light from it. The apparatus is suitable for use where checks have to be made to ensure that material being manufactured is being held constant in colour and is modified to ensure that satisfactory measurements can be made when the light transmitted or reflected is of low intensity. Fig. 1 shows a lamp 10 from which light passes through aperture 11, through one of a number of colour filters in rotatable colour wheel 13, through a further aperture 20 and lens 22, to beam splitter 23. From the beam splitter one path continues through variable attenuator 30, 32 to mirror 40 and thence to a reference standard 41 viewed by photo-cell 42. The second path passes through adjustable attenuator 31, 32<SP>1</SP> to mirror 43 and test object 44 viewed by photo-cell 45. To avoid errors that can occur in such a system when the photo-cells are subject to very low levels of illumination light from lamp 10 is also fed via mirror 50 and large aperture 51 through an aperture in disc 13 corresponding to the particular colour filter in use and differently positioned in dependence on the colour filter. The different apertures in disc 13 coincide, when in use, with respective individual attenuators such as 60 all of whose outputs feed into light guide 70 directing light on to photo-cell 45. The two photo-cells 42, 45 are arranged in a bridge circuit as shown in Fig. 4 the position of the earthy end of the measuring diagonal being adjustable as shown. Switches 81 and 82 are gauged to the colour filter wheel 13 so that a different adjustment can be preset for each colour filter. The bridge out-of-balance signal is indicated on meter A. In setting up the apparatus the test object path is first blocked. Variable aperture 30, 32 is set to cause photo-cell 42 to operate on a linear part of its characteristic and the attenuators such as 60 then set to balance the system for each of the colour filters. The setting of the aperture 30, 32 is regarded as the zero level of its calibration. The block in the test object path is then removed and a known standard placed at 44. For the first colour filter the control for aperture 30, 32 is set at the correct value for the standard and, with the bridge potentiometer set centrally, the aperture 31, 32<SP>1</SP> is set to balance the bridge. If desired fine adjustment to achieve balance may be made with the potentiometer. For subsequent colour filters the aperture 31, 32<SP>1</SP> is not touched and the appropriate bridge potentiometer is adjusted to secure a balance when the control for aperture 30, 32 is set to the correct value for the standard and the filter. As the adjustments of the attenuators such as 60 and the bridge potentiometers are not independent it is necessary to repeat the calibration process so far as they are concerned to remove any remaining errors. Then test samples may be placed at 44 and the aperture control adjusted to balance the bridge circuit for each filter in turn as required, reflectance (or transmittance) values being read off from the calibrated scale of the control.